Fix shutdown race condition

[greg-at-moderne]

What:
A follow-up to #16.
Fix a race in JsonRpc.send()/EOF handler where a request registered after the reader loop drained openRequests on peer EOF would hang until the caller’s timeout instead of failing with JsonRpcException.

Why:
Fix broken CI:

JsonRpcTest > readerLoopExitsCleanlyOnEof() FAILED
    java.lang.AssertionError: 
    Expecting a throwable with cause being an instance of:
      io.moderne.jsonrpc.JsonRpcException
    but current throwable has no cause.
    Throwable that failed the check:
    java.util.concurrent.TimeoutException